The present invention relates generally to oxidizing a semiconductor surface during an anneal processing step and, more particularly, the present invention relates to stabilizing a high pressure oxidation step using nitrous oxide gas within a temperature range of 600° C. to 750° C.
Advanced semiconductor devices, such as high density dynamic random access memories (“DRAMs”), impose severe restrictions on the times, temperatures, and atmospheres of all thermal process steps. DRAMs are comprised of a plurality of memory cells. Each memory cell is further comprised of a field effect transistor and a capacitor. It is well known in the art of semiconductor fabrication to use planar capacitors within DRAM cells; however, in DRAM cells that utilize conventional planar capacitors, more integrated circuit surface area is dedicated to the planar capacitor than to the field effect transistor.
As the density of components in integrated circuit memories increased, the shrinkage of memory cell size resulted in a number of other problems in addition to the problems associated with a smaller capacitor. Among the resulting problems was that of dopant diffusing out of the semiconductor material when forming the transistors of the memory cells. In order to form transistors, dopants must be implanted in regions of the semiconductor materials. The dopant, however, tends to diffuse out of the transistor regions when the transistors are heated during subsequent integrated circuit processing steps. For example, dopant diffuses from the semiconductor material during the reoxidation anneal of the dielectric layer of the cell capacitor.
Silicon nitride is used as a dielectric layer because it has less desirable leakage current properties than silicon dioxide. Further, a thin oxide layer is grown upon the dielectric layer by reoxidizing a layer of silicon nitride enough to form this oxide layer to further reduce the leakage current of the silicon nitride film.
Once the proper amount of silicon oxide and nitride oxide have been grown upon the surface to form the dielectric layer, a reoxidation anneal step is necessary to reduce the imperfections typically occurring during the initial reoxidation growth stages.
One method to provide the silicon dioxide film is to perform a high pressure chemical vapor deposition (HPCVD) process step on the semiconductor device. The formation of the cell dielectric, as well as transistor gate oxides and reoxidation steps in other processing application steps, is subjected to high pressures in excess of one atmosphere, typically between five (5) atmospheres to twenty-five (25) atmospheres, where an atmosphere is represented as a pressure of 760 Torr. An atmosphere of pure N2O is introduced under such pressures in a temperature range of 600° C. to 800° C. The desired reaction is:N2O→N2+O; 2N2O→2NO+N2This allows the oxygen to react with the silicon surface, forming the silicon dioxide layer.
Unfortunately, as the N2O reaction proceeds, it can become uncontrollable under certain circumstances; specifically, the N2O reaction can become supercritical, which gives rise to high pressure spikes within the high pressure oxidation furnace. These high pressure spikes abort the high pressure furnace runs and prevent the furnaces from operating in pure N2O in the temperature range of 600° C. to 750° C. As the concentration of unreacted N2O builds up in the high pressure oxidation furnace, it reaches a critical point where the disassociation reaction is self-propitiating. This reaction goes from:2N2O→2NO+N2Once the concentration of unreacted N2O exceeds this critical point, the uncontrolled reaction occurs and generates pressure spikes that may explode a furnace tube of the high pressure oxidation furnace. An exploding furnace tube results in ruined product as well as dangerous working environment conditions for personnel.
Accordingly, a method and apparatus are needed that reduce, if not prevent, the unreacted N2O from becoming super critical to ensure the uniform processing of the semiconductor wafers.